Bar code scanners and other optical devices use focused beams of light. For example, in one type of bar code scanner, a beam of light from a laser diode is projected along an optical axis and focused to a spot of light at a focal location in a working region along the axis. A moving mirror positioned in the path of the beam sweeps the spot along a line of motion within a working region. A photodetector detects light reflected from the working region and converts the reflected light to an electrical signal. If an object bearing a pattern of light and dark regions such as a typical bar code is placed in the working region so that the moving spot sweeps across these regions, the reflected light will vary in a pattern corresponding to the pattern of light and dark regions. The electrical signal will vary in the same manner. The electrical signal can be detected and decoded to yield the information stored in the bar code.
The size and shape of the spot of light will affect the scanning operation. Typically, the light which forms the spot has non-uniform intensity, which decreases gradually adjacent the periphery of the spot. The area in which the intensity exceeds a selected value, such as a selected proportion of the maximum intensity, is regarded as the spot. If the spot is too large in relation to the light and dark features to be detected, the light will illuminate both light and dark features simultaneously with substantial intensity, and hence it will be impossible to read the bar code properly. In many cases, a non-circular spot is desirable. Typically, the spot has a long dimension transverse to the direction of motion of the spot and a short dimension in the direction of motion. For example, this arrangement can be used in scanners which are intended to read a conventional one-dimensional bar code consisting of a pattern of light and dark strips extending parallel to one another.
In some bar code scanning applicatons, the distance between the scanner and the object bearing the code may vary from object to object. For these applications, it is important to provide a light beam with a substantial depth of field, i.e., a beam which has relatively small dimensions throughout a substantial range of locations around the nominal focal location. In other applications, it is more important to assure that the beam focuses to the minimum breadth at the nominal focal location, to provide the smallest beam xe2x80x9cwaistxe2x80x9d and hence provide the smallest spot size at the nominal focal location.
Typical light source apparatus used heretofore has incorporated a laser diode, a collimating lens for focusing the light from the diode to a spot at a focal location, and an opaque plate with an aperture disposed between the lens and the focal spot for blocking light at the periphery of the beam. The characteristics of the beam depend upon the characteristics of the aperture. For example, a non-circular aperture may be used to form a non-circular beam. A large aperture yields a beam with a low xe2x80x9cf-numberxe2x80x9d which is sharply focused to a small size at the nominal focal location but which increases in size rapidly with distance from the nominal focal location, i.e., a beam with good resolution but poor depth of field A narrow aperture yields a beam with a high f-number which has a somewhat larger spot size at the nominal focal location but which increases in size more slowly with distance from the nominal focal location. Such a beam has relatively poor resolution but good depth of field. Systems of this type are disclosed, for example, in U.S. Pat. Nos. 4,816,660 and 5,247,162.
There has been a need heretofore for further improvements in light beam sources and in scanning apparatus incorporating the same.
The present invention addresses these needs.
One aspect of the invention provides illumination apparatus for forming an output light beam. The apparatus according to this aspect of the invention desirably includes a light source adapted to emit a source beam of polarized light in a downstream direction along an optical axis, the source beam having a polarization direction transverse to the axis. The apparatus also preferably includes one or more polarization-altering elements disposed downstream from said source. The one or more polarization-altering elements are operative to alter the polarization of the source beam nonuniformly so as to form an altered beam having a first portion with a first polarization direction and a second portion having a second polarization direction different from the first polarization direction. Preferably, the second polarization direction is perpendicular to the first polarization direction. The first portion of the altered beam has a first intensity distribution relative to the optical axis, and the second portion has a second intensity distribution relative to this axis which preferably is different from the first intensity distribution. For example, the polarization-altering element may be a birefringent element having a hole aligned with the optical axis. Light passing through the hole constitutes the first portion of the beam, and has a relatively narrow intensity distribution with maximum intensity near the axis and substantially excluding light outside of a central zone close to said axis. Light passing through the region of the birefringent element which surrounds the hole forms the second portion of the beam and has a second intensity distribution which includes substantial light outside of the central zone.
The apparatus desirably includes one or more polarization-selective elements disposed along the optical axis downstream from the one or more polarization-altering elements. Each polarization-selective element has a transmission axis and is operative to allow transmission of light having a polarization direction parallel to the transmission axis of such element and to block transmission of light having a polarization direction perpendicular to the transmission axis of such element. The one or more polarization-selective elements may include a single polarizer. If the transmission axis of the polarizer is aligned with the polarization direction of the first portion of the altered beam, an output beam passing downstream along the optical axis from the polarizer will consist essentially of the light in the first portion of the beam. In the example given above, where the intensity distribution of the first portion is narrow, the output beam will form a spot having a high effective f-number. Such a beam will resemble the beam formed by a small aperture; it will have a relatively large spot size at the nominal focus, but will have a relatively large depth of field, so that the spot size increases slowly with distance from the nominal focus.
One or more of the elements in the system, such as the light source, the polarization-altering element, and the polarization-selective element may be movable or otherwise adjustable so as to vary the effect of these elements during operation, and thus change the configuration of the output beam during operation. Thus, the effective f-number of the output beam will vary dynamically. For example, in a system where the polarization-selective element has a transmission axis parallel to the polarization direction of the source beam, the polarization-altering element can be temporarily disabled. In this condition, the output beam will include essentially all of the light in the square beam. The output beam will form a spot having a low effective f-number. The spot size will be relatively small at the nominal focus of the beam, but will increase rapidly with distance from the nominal focus. When the polarization-altering element is enabled, the beam returns to a high effective f-number.
A further aspect of the invention provides scanners incorporating illumination apparatus as discussed above. A scanner according to this aspect of the invention desirably includes a frame and an illumination apparatus as discussed above which is mounted to the frame so that the output beam from the illumination apparatus will be directed from the frame into a working region. The scanner desirably also includes a photodetector for receiving light returned from the working region and producing a signal representing the amplitude of the returned light. The scanner may include means such as a movable or variable optical element for moving the output beam relative to the frame.
Yet another aspect of the invention provides methods of forming an output light beam. The method according to this aspect of the invention desirably includes the steps of directing a source beam of polarized light in a downstream direction along an axis and nonuniformly altering the polarization direction of the source beam to provide an altered beam having first and second portions with different polarization directions and with different intensity distributions relative to the axis. Most preferably, the method includes the step of passing the altered beam through a polarization-selective element having a transmission axis so as to allow light having a polarization direction matching the transmission axis of such element to pass downstream into an output beam, while substantially excluding light with a polarization direction perpendicular to the transmission axis from the output beam. Methods according to this aspect of the invention can be used to form output beams having the characteristics discussed above in connection with the apparatus.
An additional aspect of the invention provides methods of scanning information-bearing elements as, for example, bar-coded objects. The method according to this aspect of the invention desirably includes the steps of providing a scanning beam focused to a spot at a focal location; moving the scanning beam relative to the information-bearing elements and detecting light reflected from the information-bearing elements to provide a signal representing the information carried by such elements. Most preferably, the method further includes the step of repeatedly varying the spot size and depth of field of the scanning beam during the moving and detecting steps so as to vary the beam characteristics between a first condition in which the beam has a small spot size at the focal location but a small depth of field and a second condition in which the beam has a larger spot size at said focal location but a larger depth of field. The varying step desirably is performed so that each individual information-bearing element will be scanned by the beam in both of said conditions. Some information-bearing elements will be read best by the beam in the first condition, whereas other information-bearing elements will be read by the beam in the second condition. As further discussed below, preferred methods in accordance with this aspect of the invention can provide scanning performance superior to that achievable with a scanning beam of fixed configuration. The step of varying the beam configuration can be performed using apparatus and methods according to the foregoing aspects of the invention.